The long range objective of this project is a complete understanding, at the molecular level, of contact activation of blood coagulation and the factors affecting it. This involves educidation of the parameters of all binding interactions involving high molecular weight kininogen and other proteins and surfaces; the mechanism of autoactivation of factor XII and factors affecting it; the role of high molecular weight kininogen and surfaces on the kinetic parameters for reciprocal activation of factor XII and prekallikrein as well as the activation of factor XI. Particular attention will be paid to the role of divalent cations as cofactors in surface-catalyzed reactions. In addition, regulation of contact activation by inhibitors, and the kinetic parameters for generation of bradykinin and plasmin in the presence of surfaces, will be evaluated. These studies will use fluorescence polarization, fluorescence resonance energy transfer, enzyme kinetics and rapid reaction kinetic techniques. Quantitative data will be obtained for the stoichiometries, affinities and rates of the various interactions involved, using purified systems as well as normal and deficient plasmas. It is anticipated that this project will result in enhanced understanding of the molecular mechanisms of contact activation and the factors which affect it, as well as the relative importance of each factor XII-dependent pathway. Elucidation of these mechanisms can provide basic information on the cause of thromboembolytic disorders as well as other pathologic states affected by the proteins of the contact activation system.